1. Field of the Invention
The invention relates to a mono-condylar knee joint prosthesis.
2. Description of the Prior Art
Mono-condylar knee joint prostheses are used where it is not the entire joint connection of the knee joint, that is, the medial as well as the lateral joint connection, that is no longer intact, but rather only one half (either the medial or the lateral half), but where however the ligaments (collateral ligaments and cruciate ligaments) are still fully intact. In these cases a bi-condylar prosthesis need not be implanted. In this way the bone substance which is present can be spared as far as possible and the ligaments as well can be maintained as far as possible.
A mono-condylar knee joint prosthesis of this kind typically comprises a femur part which has a convex running surface and which is fixed either cement-lessly or cementedly at the femur, which has previously been prepared at one side. Furthermore, a knee joint prosthesis of this kind comprises a tibia part which is fixed correspondingly at the tibia, which has previously been prepared at one side. Between the femur part, which is fixed at the femur and which is typically made of metal, and the tibia part, which is fixed at the tibia and which is likewise typically made of metal, an intermediate part is typically arranged which is typically manufactured of plastic (e.g. of ultra high molecular polyethylene) and is either arranged to be slidingly displaceable on a sliding surface of the tibia part or is arranged to be non-displaceable, that is, fixed, relative to the tibia part.
In a known mono-condylar knee joint prosthesis, such as e.g. is described in U.S. Pat. No. 4,085,466, the intermediate part is substantially freely displaceably arranged on the sliding surface of the tibia part. In a knee joint prosthesis of this kind the entire guiding of the movement must be taken over by the muscle and ligament apparatus. The limiting of the movability of the intermediate part then takes place only through the muscles and the ligaments, or, respectively, through the soft parts. Since no lateral limiting of the movability of the intermediate part is present, forces in the medial/lateral direction which were previously partly taken up by the menisci or by the bone bearing respectively must likewise be taken up by the muscles and ligaments or by the still intact bone bearing respectively.
In another exemplary embodiment of U.S. Pat. No. 4,085,466 a boundary surface can be provided which laterally bounds the sliding surface. Thus e.g. in a tibia part of the lateral half of the joint connection in the region of the medial boundary of the tibia part (that is, in the region of the eminentia intercondylaris) a boundary surface can be provided at the tibia part which laterally bounds the sliding surface of the tibia part. This boundary surface extends rectilinearly in the anterior/posterior direction and serves as an abutment surface for that side surface of the intermediate part which faces the boundary surface of the tibia part, or, respectively, it serves as a guiding surface along which the intermediate part is guided. This abutment surface can thus take up forces both in the medial (or, respectively, in a replacement of the medial half, in the lateral) direction and also guide the intermediate part in the anterior/posterior direction.
The kinematics of the natural knee joint should not be influenced or modified wherever possible through a mono-condylar knee joint prosthesis. During the implantation of a mono-condylar knee joint prosthesis the articular cartilage and the meniscus are replaced (by the tibia part and the femur part and by the intermediate part respectively); the connection between the meniscus and the bone, which is present in the natural knee joint, is however not present after the implantation since in the mono-condylar knee joint prosthesis the intermediate part is movable without a connection to the tibia part. A guiding is therefore required for the intermediate part in order to be able to follow the physiological displacing of the condyles.
In accordance with more recent recognitions on the kinematics of the human knee joint it is however the case that the femoro-tibial contact point describes a path which cannot be particularly well approximated by a rectilinear pathxe2x80x94such as is described above. This means that if the actual path of the femoro-tibial contact point is approximated by a rectilinear path, as is done in the above-described guiding along the boundary surface which extends rectilinearly in the anterior/posterior direction, then the ligament apparatus is stressed more strongly than is the case in the natural, intact knee.
Here the invention wishes to provide a remedy. It is therefore an object of the invention to propose a mono-condylar knee joint prosthesis which does not have the above-described disadvantages. The prosthesis should on the one hand bring about a guided movement which corresponds as well as possible to the natural movement of the knee, and the muscles and ligaments should be stressed as naturally as possible. On the other hand the prosthesis should also be able to take up forces in the medial/lateral direction so that such forces can be taken up by the prosthesis and then ultimately by the bone, that is, by the tibia, and need not additionally be taken up by the muscles and ligaments or by the intact half of the bone bearing respectively.
The mono-condylar knee joint prosthesis in accordance with the invention thus comprises a femur part having a convex running surface. It further comprises a tibia part with a sliding surface which faces the femur part and an intermediate part which is introduced between the femur part and the tibia part during the implantation. The intermediate part has two bearing surfaces, namely a first, concave bearing surface for cooperating with the convex running surface of the femur part as well as a second bearing surface for cooperating with the sliding surface of the tibia part. A boundary surface is provided at the tibia part which laterally bounds the sliding surface of the tibia part and projects pointing in the direction towards the femur part in order to be able to cooperate with a side surface of the intermediate part. The boundary surface which is provided at the tibia part is formed to be curved in the shape of a spherical segment and the intermediate part has a side surface which is curved corresponding to the boundary surface which is provided at the tibia part and which is curved in the shape of an arch.
In a preferred exemplary embodiment the boundary surface which is provided at the tibia part is formed to be curved in the shape of a circular segment, and the side surface of the intermediate part is accordingly formed to be curved in the shape of a circular segment.
The boundary surface, which is arcuate, in particular curved in the shape of a circular segment, and the side surface of the intermediate part, which is curved corresponding to this boundary surface, represent a very good approximation to the natural path of the femoro-tibial contact point, through which the muscles and ligaments are stressed similarly as in the natural, intact knee. In addition, forces acting in the lateral/medial direction can be taken up by the prosthesis or ultimately by the tibia respectively and need not be taken up by the muscle and ligament apparatus as in a prosthesis with a freely movable intermediate part.
In a further advantageous exemplary embodiment of the mono-condylar knee joint prosthesis in accordance with the invention the boundary surface which is provided at the tibia part is formed to be convex and the corresponding side surface of the intermediate part is formed to be concave. This is advantageous because the lateral condyle travels through the significantly longer path in a flexion/extension movement than the medial condyle, and the path of the femoro-tibial contact point of the lateral condyle extends particularly well through a movement along a path which is convex, in particular has the convex shape of a circular segment.
In a further advantageous exemplary embodiment the radius of curvature of the boundary surface of the tibia part and accordingly the radius of curvature of the side surface of the intermediate part lies in the range from 30-50 mm. This range comprises the essential range of the radii with which a movement of the femoro-tibial contact point such as is present in the natural knee is approximated.
In a further advantageous exemplary embodiment the width of the tibia part lies in the range from 15-55 mm and the length of the tibia part lies in the range from 35-60 mm. These ranges for the dimensions of the width and length of the tibia part cover the dimensions of the tibia most frequently occurring in humans.
Finally, in a further advantageous exemplary embodiment the arcuate side surface of the intermediate part extends over an angle of arc which lies in the range from 25xc2x0-45xc2x0. It is thereby ensured that the meniscus part has a sufficient size in order to ensure a sufficiently large contact surface between the running surface of the femur part and the bearing surface of the intermediate part.